(i) Field of the Invention
The present invention relates to a method for preparing molded articles of high-purity alumina fibers having high heat resistance which are suitable for high-temperature insulating materials, catalyst carriers and the like.
(ii) Description of the Prior Art
Alumina fibers are materials having high strength and excellent heat resistance, and they are used in a cotton-like state or in the form of molded articles. In particular, molded articles obtained by molding the alumina fibers together with various binder components and matrix components have small bulk density and so they are lightweight. In addition, they have a large insulating effect, and therefore they are suitable for high-temperature insulating materials.
As a method for preparing the molded articles of these alumina fibers, there are known methods which comprise cutting alumina, alumina-silica or alumina-silicaboron oxide ceramic fibers into a suitable length; adding an inorganic binder such as silica fibers, an alumina sol or a silica sol, or an organic binder thereto; and then molding and drying the mixture (e.g., Japanese Patent Application Laid-open Nos. 59-152281, 61-141683 and 64-42373, and other publications). When the molded articles manufactured by these known methods are used at a high temperature, they are to be calcined at a suitable temperature to carry out a binder removal treatment, prior to their use. In short, the above-mentioned known techniques require the employment of the calcined fibers as the raw material to obtain desired strength, heat resistance and the like as well as the binder to supplement the sintering properties of the fibers as an essential component.
However, in the molded articles of alumina, aluminasilica or alumina-silica-boron oxide ceramic fibers prepared by these methods, cracks easily occur as a result of heat shrinkage, when the ceramic fibers calcined at a temperature of 1000.degree. C. or less are used, and therefore they have the substantial problem of heat resistance. On the other hand, when the fibers are obtained by the calcination at 1000.degree. C. or more, a large amount of energy is consumed. When an alumina content is low, heat resistance, strength and the like cannot be improved, and therefore, in such a case, raw materials are limited. Furthermore, when alumina fibers obtained by the calcination at a temperature of 1000.degree. C. or more are used as the raw material, the molded articles of the high-purity alumina fibers can be obtained However, the sintering properties of the fibers themselves are poor, and therefore even if the binder is added thereto, sintering between the fibers is insufficient. In consequence, even if the strength of the fibers is sufficient, the molded articles obtained therefrom have low strength, and crystallization of the fibers, when used, proceeds at a high temperature, so that the fibers tend to fall from the surfaces of the molded articles.
Moreover, another method has been suggested in which precursor fibers of alumina, silica, zirconia or the like are molded and then calcined to obtain a fibrous refractory having high strength. In this case, the consumption of energy can be decreased at the time of the manufacture (Japanese Patent Publication No. 25873/1990). However, in this method, the precursor fibers before the molding adhere to each other during storage, so that it is difficult to disperse them in a fibrous state in a solvent when they are molded. This fact makes it difficult to stably manufacture the uniformly molded articles having high strength at times. In addition, this kind of precursor fibers noticeably shrink at the time of calcination, and therefore they have the problem that the molded articles obtained therefrom are likely to crack.
The problems of the above mentioend technology can be summarized as follows: When the fibers calcined at 1000.degree. C. or more are used as the raw material, sufficient heat resistance and strength cannot be obtained, unless the alumina has a high purity. On the other hand, when the high-purity alumina is used, the sintering properties are insufficient, so that strength deteriorates and the fibers tend to peel from the surfaces of the molded articles. Furthermore, since the fibers calcined at 1000.degree. C. or more are used, a large amount of energy is consumed during the manufacture. Moreover, when the precursor fibers are used as the raw material, it is possible to obtain the molded articles having excellent strength and heat resistance under the small energy consumption. However, it is not easy to handle the precursor fibers, and these precursor fibers adhere to each other during storage and the molded articles cannot be obtained on occasion.
In view of the above mentioned situations of the conventional techniques, an object of the present invention is to provide a method for stably preparing molded articles of high-purity alumina fibers which are light-weight and have high strength and excellent heat resistance. The above-mentioned molded articles are extremely useful as high-temperature insulating materials, base materials for various composite materials, catalyst carriers and the like.